Stockholm police are investigating what drove a Swedish cleaning woman to crash an empty commuter train into an apartment complex in a suburb of Stockholm.

"The woman stole an empty commuter train from a train depot and drove it to a suburb of Stockholm, where it derailed and slammed into an apartment building," a Stockholm police spokesman told the U.K.'s Telegraph.

Police arrested the woman, who was 20, on suspicion of endangering the public.

Although no one else was hurt, the woman was taken to a Stockholm hospital with serious injuries.

"The woman stole the four-car train at a depot outside Stockholm," Tomas Hedenius, spokesman for Arriva train operator, was quoted as saying in the U.K.'s Independent. "She then drove it about a mile to the end station on the railway line, where it jumped off the tracks, careered for about 30 yards and crashed into a three-story building."

Hedenius said "three families were inside the apartment building, but no one was injured. At least not physically."

It's unclear how the woman, employed by a cleaning company that Arriva outsourced to, was able to get the keys to the train. Hedenius said that "driving it [the train] is not that complicated."